1. Field of the Invention
The present invention relates to a water-soluble polyimide precursor, an aqueous polyimide precursor solution and a polyimide and, particularly, it relates to a water-soluble polyimide precursor, an aqueous polyimide precursor solution and a polyimide which give polyimide molded articles that maintain a high level of heat resistance while exhibiting high tensile strength and high elongation.
The invention further relates to an impregnated material with a polyimide binder and to a laminate, and particularly it relates to an impregnated material with a polyimide as the binder which can be produced substantially without using any organic solvent in the heating step and which ensures heat resistance and strength for molded articles, and to a laminate suitable for use as a circuit board.
2. Description of the Related Art
Polyimides obtained by reaction between aromatic tetracarboxylic dianhydrides and aromatic diamines generally exhibit excellent heat resistance, mechanical strength, electrical properties and solvent resistance, and are therefore widely used in the electrical and electronic industrial fields. However, most fully aromatic polyimides have poor solubility in organic solvents and, consequently, an organic solvent solution of the precursor polyamic acid is usually applied and subjected to dehydrating ring closure, by high temperature heating, for preparation of a polyimide molded article. For this reason they have not always been advantageous from the standpoint of working environment, and their uses have therefore been limited.
As polyimide molded articles using no organic solvents there are widely employed molded articles molded from pyromellitic acid-based polyimide powders obtained from pyromellitic acid components and 4,4′-diaminodiphenylether.
Polyimide powders, however, have no appropriate solvents in which they can dissolve, and therefore their uses have been limited due to considerations of molded article production and workability.
Water-soluble polyimide precursor powders and aqueous polyimide precursor solutions have therefore been proposed.
In connection with such water-soluble polyimide precursor powders and aqueous polyimide precursor solutions, Japanese Examined Patent Publication No. 3-15659, for example, describes an example of obtaining a water-soluble polyimide by synthesizing a 2,3,5-tricarboxy-cyclopentylacetic acid-based polyimide precursor in an amide-based solvent and reacting it with triethylamine, diethylamine or the like.
Also, Japanese Unexamined Patent Publications No. 8-3445, No. 8-59832 and No. 8-291252 describe examples of obtaining water-soluble polyimide precursors by reacting aminoalcohol-based amine compounds with polyimide precursors.
However, the aforementioned water-soluble polyimide precursor described in Japanese Examined Patent Publication No. 3-15659 has a special chemical structure which limits its performance and application.
In addition, the water-soluble polyimide precursors described in the aforementioned Japanese Unexamined Patent Publications No. 8-3445, No. 8-59832 and No. 8-291252 have limited applications because the polyimide films prepared as molded articles therefrom have low heat resistance (especially thermal decomposition temperature) and mechanical properties (especially elongation) compared to polyimide films obtained from polyimide precursors employing ordinary polar organic solvents.
Nonwoven fabrics comprising aramid fibers fixed with binders made of phenol resins, epoxy resins or thermoplastic polyesters have been developed as heat-resistant nonwoven fabrics and, for example, Japanese Unexamined Patent Publication No. 10-131017 describes fabrication of a circuit board with aramid fibers and an epoxy resin; however, because of the inadequate heat resistance of the binder, the board has not been suitable for prolonged use under high temperature conditions. Heat-resistant nonwoven fabrics using water-soluble polyimide varnishes and the like as binders have also been developed, but these have also been impractical because of vastly reduced strength under environments at 200° C. or above. Although the heat resistance of the fibers composing these heat-resistant nonwoven fabrics is adequate, the unsatisfactory heat resistance of the binders has been a major problem.